Yeah, River is the pretty obvious guess. I don't think that will stop people from speculating like crazy though.

What makes it especially frustrating--but in a fun way--is Moffat's doing so much story wise with clues/red herrings and mixing in so much real world 50th stuff into it. For example, the ages missing from her book: 16 and 23. Sixteen is the number of years between the old series and the current one. Twenty-three is the air date for the 50th. And, fuck it, throw in an eleven and you fucking have fifty. Who the fuck knows?!?

As for the upcoming episodes, I can't wait to see Gaiman's take on the Cybermen. Sounds like he is going to do the full on return of the original Mondas Cybermen from the rumors. Whatever the case, I am quite hopeful when Moffat hands Gaiman the Cybermen and says to make them scary again.